Saturday, March 8, 2008

Stress versus Ductility

It is important to differentiate between the deleterious effect of high stress and lack of ductility. There are no predictable relationships between hardness, brittleness, and stress. Relatively soft deposits from the Watts bath may be highly stressed in tension and show good ductility whereas nickel deposited in the presence of sodium naphthalene trisulfonate can show zero stress with a Vickers hardness of 540 and a low ductility.

 

Early Nickel Plating Problem

Nickel plating had about a 50-years history in the plating of polished brass and steel articles before the advent of commercial chromium plating in 1942. The nickel plate was comparatively pore free and protected the underlying metal very well. It was deposited from plain inorganic baths and the dull plate was polished. With the introduction of chromium plating, it was first thought that this thin, hard, non tarnishing plate would replace nickel plating, but it was soon found that thin chromium deposited directly on brass plumbing goods became discolored from green corrosion products formed from the brass through minute pores in the thin chromium plate. A galvanic couple forms under moist condition with the passive chromium surface becoming the cathode, and brass exposed in the pores of the chromium plate the anode, of the corrosion cell, and with the unfavorable condition of large cathodic areas and small anodic areas, pitting corrosion result. The corrosion proceed undercutting of the chromium plate, resulting in flaking off of pieces of the hard, brittle chromium and the accumulation of unsightly greenish brass corrosion products. Even to this day, brass plumbing goods are sometimes directly plated with chromium, especially in times of nickel shortages, or when manufactures do not have plating specifications. It was soon realize that applying the very thin chromium plate on top of nickel solved the tarnishing problem with nickel, and the poor corrosion protection with thin bright chromium plate alone.

 

The cost of buffing the dull nickel plate to a high luster, and the problem of cut through stimulated the search for addition agent to produce bright ductile plate directly from the plating bath.

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